Ceramic materials are currently being used successfully in a variety of molten metal applications. These applications usually involve extreme environmental conditions such as rapid rises in temperature, and severe temperature gradients. A particularly demanding application is in breakrings in horizontal continuous casters. Breakrings are commonly subjected to extremely fast temperature rises, and high temperature gradients often in excess of 1000.degree. C./cm. These conditions require a material that has good thermal shock resistance to prevent breaking. Additionally the material should preferably have a high abrasion resistance and corrosion resistance with respect to molten metals, be machinable, and be economical to manufacture.
Boron nitride is successfully used as a material for breakrings due to its good thermal shock resistance, corrosion resistance, stability at high temperature, and machinability. However, it lacks good abrasion resistance, which renders it subject to high wear rates when exposed to flowing metal. Additionally, BN ceramics typically contain a B.sub.2 O.sub.3 binder phase that can react chemically with molten metals, which further degrades the integrity of the boron nitride ceramic. The degradation of the BN can also cause problems with the metal being cast. BN particles, as well as bubbles which form from gaseous B.sub.2 O.sub.3 or CO.sub.2 from the reaction of B.sub.2 O.sub.3 with carbon, can be trapped in the metal as it solidifies.
Alumina is also used in molten metal applications due to its hardness, abrasion resistance, and chemical stability. Although satisfactory, alumina ceramics often have poor thermal shock properties, and are difficult to machine because of their hardness.
It would be desirable to provide a refractory material which has the abrasion resistance, and chemical stability of Al.sub.2 O.sub.3, but also has the thermal shock resistance, and good machinability of BN.
Rice et al. in "Thermal Structural Ceramic Composites" in Cer. Eng. Sci. Proc., 1:424-43, (Nos. 7-8,1980) disclose an alumina-boron nitride composite for dielectric applications. A fine agglomerate free BN is preferred as the source of boron nitride. The alumina and boron nitride were milled, and processed at a temperature greater than 1800.degree. C. Processing temperatures less than or equal to 1700.degree. C. are described as not suitable. The resulting composites had Young's Moduli between 193.05 and 199.95 MPa. (28 and 29 psi.times.10.sup.6)
Lewis et al. in "Microstructure and Thermomechanical Properties in Alumina- and Mullite-Boron-Nitride Particulate Ceramic-Ceramic Composites", Cer. Eng. Sci. Proc., 2:719-27, (Nos. 7-8,1981) disclose alumina/boron nitride composites made by wet milling fine boron nitride and alumina particulate materials for 16-40 hours in an alcohol, and hot pressing in air or vacuum conditions. The composites disclosed by Lewis et al. had densities from 94.5 to 96.5 % of the theoretical density, parallel Young's Moduli between 103 and 77 GPa (11.times.10.sup.6 and 17.times.10.sup.6 psi), a parallel CTE of 20.times.10.sup.-6 1/.degree. C., and a perpendicular CTE of 7.times.10.sup.-6 1/.degree. C.